1. Field of the Invention
The present invention relates to an internal combustion engine operating on a six or more stroke cycle. The combustion is divided into two parts: a first lean combustion and a second stoichiometric combustion.
2. Background Art
It is known to those skilled in the art that it is more efficient to operate an engine at an air-fuel ratio which is lean of stoichiometric (i.e., higher air-fuel ratio) rather than at stoichiometric for a comparable speed-torque condition. However, most spark-ignition engines produced, at the present, are stoichiometric engines to facilitate the high efficiency conversion of the regulated emissions: carbon monoxide, hydrocarbons, and nitrogen oxides, in an exhaust aftertreatment device. Engines operating at lean air-fuel ratios often use a reductant to reduce nitrogen oxides within an exhaust aftertreatment device, such as a lean NOx catalyst, with the reductant management and delivery system an undesirable additional requirement on the exhaust aftertreatment system.
The inventors herein have recognized a method for operating an engine with a lean combustion event without incurring the emission aftertreatment difficulties associated with lean combustion. Specifically, a first combustion event occurs without exhausting the lean products of combustion. The valves in the cylinder remain closed during an expansion stroke and a compression stroke. A fuel injector in the cylinder head provides fuel to the combustion gases in the cylinder. The amount of fuel added is an amount to bring the mixture in the combustion chamber to stoichiometry. The mixture undergoes a second combustion process and is subsequently exhausted into an aftertreatment device. Because the gases arriving at the exhaust aftertreatment device are stoichiometric, the exhaust aftertreatment device can convert nitrogen oxides without supplying a reductant.
A method of operating an internal combustion engine, in which fuel injectors are disposed in the cylinders, is disclosed. The method steps include: inducting air into a cylinder of the engine, compressing the air, providing a first amount of a fuel to the air, combusting the first amount of fuel in air by a first combustion, expanding and compressing products of the first combustion, providing a second amount of fuel to the products of the first combustion, and combusting the second amount of fuel. The first amount of fuel is an amount which when mixed with the air causes the contents of the cylinder to have an air-fuel ratio which is leaner than stoichiometric. The second amount of fuel is an amount which when mixed with the products of the first combustion causes the cylinder to have an air-fuel ratio which is substantially stoichiometric.
An advantage of the present invention is that a first combustion event may be a lean combustion event (may be stratified or homogeneous lean) without the nitrogen oxide emission aftertreatment difficulties associated with lean combustion. This is possible because the products of combustion of the lean combustion are not exhausted after the first combustion event. In the present invention, the products of the lean combustion participate in a second combustion event, which is homogeneous and stoichiometric. A three-way catalyst aftertreatment device being fed stoichiometric exhaust gases is known, by those skilled in the art, to convert the three regulated emissions: carbon monoxide, hydrocarbons, and nitrogen oxides, at high efficiency. In this way, the high efficiency of stratified combustion can be achieved at an acceptable emission level.
A further advantage of the present invention is that the second combustion event occurs within a mixture with a relatively high quantity of hot combustion products. The second combustion produces very low levels of nitrogen oxides.
Yet another advantage of the present invention is that because the second combustion event occurs in a homogeneous, stoichiometric mixture, soot production is negligible.
In an engine equipped with valves which allow fully flexible timing of valve events, load is controlled primarily by valve timing. However, at the lightest loads and speeds, it is found that stoichiometrically fuelled engines require throttling to ensure acceptable combustion stability. Throttling leads to a fuel efficiency penalty. The inventors of the present invention have recognized that by spreading the torque produced over six or more strokes, the power from the engine is reduced without throttling. Thus, the inventors have devised a way to operate the engine in a manner which results in lower torque without incurring pumping losses due to throttling.
Another advantage of the present invention is that inducted air is combusted in two events. The inventors of the present invention have recognized that two small combustion events improve engine smoothness over one large combustion event if compared over the same number of strokes. The ability to have two combustion events without opening the valves is facilitated by providing a fuel injector in communication with the cylinder head, commonly called direct injection. With a direct injection engine, fuel may be provided independently of an air intake process. In contrast, in conventional port injected engines, fuel may be supplied to the cylinder only during an intake process, that is when air carries fuel into the cylinder. The ability of a direct injection engine to supply fuel directly to the combustion chamber provides for combustion of a cylinder""s charge of fresh air occurring in two parts.